It is known that the main problem observed in using shoes with an ordinary sole made of natural material such as leather or equivalents is constituted by wet walking areas.
When rain and bad weather make streets wet and slippery, it is in fact not advisable to use shoes with leather soles, since leather, just because it is breathable and healthy for the foot, is not waterproof but in fact absorbs water.
The thinner the leather, the higher the rate at which it becomes impregnated with water or moisture, ultimately wetting the user's foot.
Accordingly, the use of soles with a leather tread is constrained by weather conditions, and for this reason shoes made of this type of material are mainly provided by manufacturers in the summer collections in countries where the dry season is more substantial.
A sole (disclosed in U.S. Pat. No. 5,598,644 and EP 0 619 959) has been devised which comprises a tread made of leather or similar material that is at least partially covered in an upward region by a membrane made of a material that is vapor-permeable and waterproof.
The waterproof and breathable membrane is fixed to the sole by means of adhesive arranged in spots, and a perimetric sealing ring made of polymeric material, for example polyurethane, is injected in a mold to assemble all the parts.
Although this sole is a considerable technical step forward, in that it renders the leather tread waterproof, it has in any case shown some limitations.
A first limitation arises from the very nature of leather, which as mentioned is a breathable material, but its breathability is not so high as to dissipate in a short time all the heat and vapor that form inside the shoe during use.
A second limitation is due to the fact that as mentioned the seal is produced by injection-molding a ring of plastic material.
This process requires the provision of thick seals and accordingly increases the rigidity of the sole, so that some types of shoe in which high flexibility is required, such as women's shoes, are difficult to manufacture.
The high cost of this technology (provision of an aluminum mold for each shoe model and size to be produced) is not inconsequential.
An attempt has been made to obviate the problem of thicknesses by using, for the perimetric seal, films made of thermoplastic polyurethane material with a thickness of 200 microns.
Although this method allows to obtain qualitatively acceptable products, it is extremely expensive, since it is almost entirely manual.